1. Field of the Invention
The invention relates to a vehicle steering system that is used in a loading vehicle such as a forklift.
2. Description of Related Art
A forklift often turns a corner having an angle close to a right angle. The rear wheels of the forklift serve as steered wheels. Therefore, at the time of turning a corner having an angle close to a right angle, a steering operation specific to the forklift is performed, that is, the front (fork portion) of the forklift is slightly turned in a turning direction at a small steering angle and then the rear of the forklift is caused to swing out toward the outer side of the corner through a quick steering operation. The forklift employs a so-called steer-by-wire power steering system in which a steering member provided in an operator cab and the rear wheels that serve as steered wheels are not mechanically connected to each other. Therefore, there is originally no reaction force applied from the rear wheels to the steering member. Accordingly, in related art, simulated reaction force is created by a device arranged on the steering side and is supplied to an operator.
In addition, a steering ratio, which is a ratio between a steering angle of a steering wheel and a steered angle of steered wheels, is set such that the steering angle and the steered angle are in a proportional relation (see Japanese Patent Application Publication No. 2010-264833 (JP 2010-264833 A).
The steering angle range of a forklift is wide and a steering operation is frequently performed in the forklift, due to the nature of its use. Therefore, an operator needs to operate a steering wheel by a large amount, so a large burden is placed on the operator. Simulated reaction force created by a device arranged on the steering side has such a characteristic that the simulated reaction force increases linearly with an increase in the steering angle until the steering angle reaches a certain value, and is maintained at a constant value after the steering angle reaches the certain value, as shown in the graph of the “characteristic in related art” in FIG. 4. Therefore, in an initial stage of a turn, reaction force is too small and steering force overcomes the reaction force. Accordingly, the steering angle is often larger than a steering angle intended by the operator. In addition, in a late stage of the turn, a quick steering operation needs to be performed. However, large force is required to perform a quick steering operation against the large reaction force. Therefore, a large burden is placed on the operator. This reduces the working efficiency.